The present invention relates to methods and apparatus for bombarding an object with charged particles, and more particularly for bombarding plastics with electrons to accomplish cross-linking.
Cross-linking is a process by which inter-molecular bonds are formed between the otherwise amorphous molecules of a polymeric plastic. The results of this process renders the plastic quasi-crystalline and therefore improved in its physical and electrical characteristics.
A chemical or electrical agent is employed to remove ("scavenge") hydrogen atoms from the hydrocarbon molecules of a polymer. The resulting open carbon valences then interlock forming inter-molecular bonds. The simplest form of cross-linking is the vulcanization of rubber using sulphur as the scavenging agent. Heating the rubber provides the energy needed to break the carbon-hydrogen atomic bonds. The sulphur, with two available valences, interposes itself between two single-valenced carbon atoms, forming a carbon-sulphur-carbon intermolecular bond, or cross-link.
For more complex hydrocarbons, organic peroxides may be used as the hydrogen scavengers. These are organic (carbon-hydrogen based) compounds containing positive free radicals which perform the same function as sulphur in the vulcanizing process. Again, heat provides the energy to break the carbon-hydrogen bond.
Electromagnetic energy in the form of alpha or beta particles, gamma rays, or even sunlight can accomplish the same ends: When plastic of sufficiently high molecular weight is exposed to any of these forms of radiation, direct carbon-carbon cross-links are obtained. Since the cross-links contain no interposing atoms (such as sulphur) the bonds thus formed are "strongest" in that they require the most energy input to "break."
Note here that regardless of the process used, free hydrogen is liberated. If allowed to concentrate, this gas presents a safety problem.
Today, most polymers are chemically cross-linked because the agent is easily dispersed in the plastic compound during the extrusion or molding process. These agents are expensive, explosive, highly toxic, and require time at elevated temperature to perform their tasks.
As mentioned above, cross-linking can be effectively accomplished using high energy electromagnetic radiation. Commonly, an electron beam (beta particles) similar to that emitted by a television picture tube is used. The kinetic energy of the electrons is sufficiently high to penetrate the polymer and then break the carbon-hydrogen bonds. Where electrons cannot be imparted with sufficient energy to perform the process, other forms of radiation having higher energies (e.g., gamma rays) are used.
As with all forms of radiation, shielding must be employed to prevent stray particles or rays from adversely affecting the general public. Large concrete structures are, therefore, employed to prevent this leakage to the environment.
At present, electron beam cross-linking is accomplished either uni- or bi-directionally in a single plane using a constant current of electrons.
U.S. Pat. No. 2,724,059 to A. J. Gale shows a standard electron gun with a flared end having shaping magnets within the flared end for directing the beam towards an object to be irradiated. The Gale magnets act as a magnetic lens to shape the beam leaving the electron gun. Gale shows the placement of two guns opposite the object to simultaneously bombard the object from both sides. U.S. Pat. No. 4,543,487 to Puumalainen, et al. shows an alternate method in which a filament is used to produce electrons, with a row of capture magnets being used to draw the electrons through the object. The power applied to the various capture magnets can be varied in order to maintain a uniform bombardment intensity despite variations in the physical properties of the filament.
U.S. Pat. No. 3,714,416 to Link, et al. shows a method for aiming the electron beam at an oblique angle of entry to the object to provide for a more uniform penetration of the electron beam into the object. U.S. Pat. No. 4,551,606 to Inoue shows a system in which the intensity of the electron beam is controlled through pulse width modulation.
These prior art devices exhibit one or more of the following disadvantages.
First, in the absence of capture magnets (such as those used in Puumalainen U.S. Pat. No. 4,543,487) the beam is, essentially, unguided, leading to "backscatter" or reflected energy leakage.
Second, the combination of constant beam current and electron capture within the plastic itself will allow, after a time, a static electrical charge to build up in the polymer. Unless drained off to ground using an auxiliary electrical conductor, this electrical potential may discharge to ground potential through the polymer resulting in pinholes. If the polymer itself is to be used as an electrical insulator, pinholes will provide an open path to ground during service, rendering the product unsafe for use.
Third, the process of using charged particles for cross-linking is most feasible for continuous objects, such as extruded wire insulation. To cross-link individual objects would require actual, physical rotation of the object under the beam to allow for uniform dose throughout the (perhaps irregular) object volume. This process is time-consuming and costly since the beam must be shut off and restarted after each rotation. In the two gun approach of Gale U.S. Pat. No. 2,724,059, the intensity must be limited to prevent interference caused by the beams going all the way through the object.
In other areas of technology, magnets are used to control charged particles for other purposes. For instance, in the bombardment of silicon wafers with charged particles, a charged particle gun will produce a beam which can have its direction changed by magnets to allow mounting the gun below the wafer and directing the beam to impinge upon the wafer from above. Additionally, magnets may be used to focus charged particles onto the silicon wafer. One such focusing mechanism is shown in U.S. Pat. No. 4,366,383 to Sano, et al.
Another technology in which magnets are used in conjunction with charged particles is the field of nuclear fusion reactors. Here, containment magnets are used to prevent the fusion plasma and the by-products of the fusion process from escaping the system.